Vibrational Stresses of Damaged Steam Turbine Blades After Renovation Repair

Authors

Mykola H. Shulzhenko, A. Pidhornyi Institute of Mechanical Engineering Problems of NASU (2/10, Pozharskyi St., Kharkiv, 61046, Ukraine), e-mail: mklshulzhenko@gmail.com, ORCID: 0000-0002-1386-0988

Anton S. Olkhovskyi, A. Pidhornyi Institute of Mechanical Engineering Problems of NASU (2/10, Pozharskyi St., Kharkiv, 61046, Ukraine), e-mail: tosky94@gmail.com, ORCID: 0000-0001-5741-4990

Abstract

The last-stage blades of K-1000-60 / 3000 steam turbines operate in a humid steam environment, which causes erosion damage in the blades and reduction in their residual life. The relevance of this work is related to the need to continue the safe operation of such turbine blades. A number of variants of the finite-element models of individual blades and last-stage blades in the disk-blade systems of the above turbines are considered. Results of the numerical study of the influence of blade part removals in erosion damage zones after renovation repair on the vibration characteristics of individual blades and blades in the disk-blade system are presented. An analysis of the stress-strain state under the conditional load from the steam flow during the forced oscillations of individual blades and blades in the disk-blade system is carried out. The loads are given as evenly distributed and linearly variable on blade surfaces. The dependence of the maximum equivalent vibration stresses on excitation frequency is determined. It is assumed that the physical and mechanical properties of the blade material are preserved (as for the original version) after the renovation repair of blades and processing of their surfaces. There is a significantly greater reduction in the vibration stresses of blades in the disk-blade system than in the stresses of individual blades. Graphs of the dependence of the maximum stresses on excitation frequency both for undamaged individual blades and blades in the disk-blade system after their renovation repair are given. Various variants of blade part removals in areas of blade leading and trailing edges are considered. It is shown that with decreasing chords of blades after renovation repair, frequency regions of increased vibration may appear in lower blade parts. In the lower parts of individual blades and blades in the disk-blade system, the maximum stresses increase in comparison with their values in undamaged blades. With the change in the stress-strain state of rotor blades in comparison with the original version of undamaged blades, the possibility of extending their safe lifetime in case of multi-cycle fatigue is assessed. The safe lifetime of the considered blades with a chord of at least 150 mm after their renovation repair can be extended according to their stresses, if the cyclic symmetry of the disk-blade system is not violated, and the physical and mechanical properties of the material are preserved after the processing of damage removal zones on blade trailing edges.

Keywords: blade, vibrations, erosion damage, renovation repair, lifetime extension, three-dimensional finite element model, disk-blade system, forced oscillations, amplitude-frequency characteristic.

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References

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Received 11 February 2021

Published 30 March 2021